1. Field of the Invention
This invention relates generally to an apparatus for determining a position of a rotating member of an electric machine, and, more particularly, to an apparatus for accurately determining the position having a pulse switching strategy.
2. Disclosure of Related Art
For conventional variable frequency induction machines (i.e., electric induction motors), it is desirable to determine incremental angular position of the rotor. The incremental position is used to control the stator electrical excitation frequency so as to maintain a desired slip frequency (i.e., the difference between the rotor speed or frequency and the applied stator excitation frequency). As known, failure to maintain the proper slip frequency will result in a loss of torque production as well as a loss in efficiency.
A number of position determining approaches are known. For example, it is known to employ a rotating, ferromagnetic target wheel and a sensor to determine position. Exemplary of this approach is seen by reference to U.S. Pat. No. 5,754,042 entitled "MAGNETORESISTIVE ENCODER FOR TRACKING THE ANGULAR POSITION OF A ROTATING FERROMAGNETIC TARGET WHEEL" issued to Schroeder et al. Schroeder et al. disclose a target wheel having a plurality of teeth, separated by slots, angularly spaced around the periphery thereof. Schroeder et al. further disclose two magnetoresistive (MR) sensors positioned adjacent the target wheel, each generating a signal with transitions between two voltage levels at the passage of the leading and trailing edges of a tooth. As applied to induction machines, it is further known to use two sensors (fixed MR or Hall effect) in quadrature (i.e., the sensors are spaced apart a distance equal to one-half tooth). The state transitions or "edges" of the sensor output signal, which correspond to the leading and trailing edges of the tooth as it passes the sensor, are counted by a controller to calculate incremental position.
As further background, it is a characteristic of induction machines to require the highest incremental position resolution at relatively low speeds, when the time between successive pulses is longest, and the acceleration rate of the machine is typically the greatest. At high rotational speeds, the time between successive incremental position pulses is short, relative to the rotor acceleration rate, and thus the resultant speed error given acceleration over time is relatively low. Hence a lower resolution encoder is allowable. However, during startup or at low speeds, a resultant speed error given acceleration over time becomes unacceptably large, which limits the torque production of the induction machine and the response time of the system including the machine.
One approach therefore, taken in the art, is to simply provide a single, high resolution sensing system to accommodate the high-resolution requirements of relatively low speed operation. However, there are shortcomings to this approach. For example, optical encoders, if used, provide high resolution, but are costly. Alternatively, the target wheel in a sensor/target wheel approach can be made to have more teeth, thereby increasing resolution. There are, however, several factors limiting the number of teeth that can be produced on the target wheel (i.e., the greater number of teeth, the higher the resolution). First, as to manufacturing costs, tolerance requirements for a very high tooth-count target wheel and the increased time to form such a wheel add to the overall cost of the system. Second, magnetic design constraints limit the number of teeth that can be sensed on a target wheel of a given diameter. That is, proper operation of the sensors requires a minimum spacing between teeth. Finally, there are electrical concerns. The sensor devices conventionally used have predetermined rise and fall times (i.e., for the sensor output signal to transition between low and high states when passing leading and trailing edges of a tooth). At low speeds, such rise and fall times are generally inconsequential. However, at higher speeds, the rise and fall times interfere in applications where two sensors are used in quadrature. That is, at high speeds, the rise and fall times may causes the fall of the first sensor output signal to substantially coincide in time with the rise of a second, adjacent sensor output signal, rather than be spaced apart by one-half tooth (e.g., in quadrature). This can confuse edge detection circuitry into only recognizing one "edge" when two "edges" should be counted. This situation leads to errors in determining incremental position. Thus, rise and fall times, tolerances, and duty cycle also limit the number of teeth on the target wheel.
There is therefore a need to provide an apparatus for position determination that minimizes or eliminates one or more of the shortcomings as set forth above.